The present invention relates to a process of fabricating a semiconductor device, particularly a semiconductor IC having a MOS transistor formed on a polysilicon layer, which in turn is on an insulating layer.
In prior art semiconductor IC devices, active elements such as transistors are formed in a single crystal silicon substrate and these elements are connected by wiring. However, to enable a further increase in the degree of integration, there is an increasing demand for realizing a multi-stack LSI or a three-dimensional LSI in which transistors are also formed on an insulating layer. Various methods have been studied to meet the demand. In one method, polysilicon is deposited on an insulating layer, and the deposited polysilicon is changed into single crystal silicon by LASER irradiation. In another method, the polysilicon itself is used for the formation of the transistor.
However, the LASER irradiation technique is not yet fully developed or controllable and transistors formed on polysilicon have poor characteristics. This is because the polysilicon layer formed by CVD consists of minute grains (normally about 10 to 100 nm) and there are plenty of electrically active sites where interatomic bond are disconnected, which are called dangling bonds, or misfits of crystals present in the grain boundaries, causing degradation of the transistor characteristics. Measures to alleviate the problem are now being studied. In one measure, the polysilicon is heat-treated in hydrogen atmosphere to inactivate, electrically, the dangling bonds.
The above-described measure has not yet been successful in fully improving the characteristics of the transistors.